Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/examples/class_Block.cpp

@@ -0,0 +1,24 @@
+#include <Eigen/Core>
+#include <iostream>
+using namespace Eigen;
+using namespace std;
+
+template <typename Derived>
+Eigen::Block<Derived> topLeftCorner(MatrixBase<Derived>& m, int rows,
+                                    int cols) {
+  return Eigen::Block<Derived>(m.derived(), 0, 0, rows, cols);
+}
+
+template <typename Derived>
+const Eigen::Block<const Derived> topLeftCorner(const MatrixBase<Derived>& m,
+                                                int rows, int cols) {
+  return Eigen::Block<const Derived>(m.derived(), 0, 0, rows, cols);
+}
+
+int main(int, char**) {
+  Matrix4d m = Matrix4d::Identity();
+  cout << topLeftCorner(4 * m, 2, 3) << endl;  // calls the const version
+  topLeftCorner(m, 2, 3) *= 5;                 // calls the non-const version
+  cout << "Now the matrix m is:" << endl << m << endl;
+  return 0;
+}